1. Field of the Invention
The invention relates to the determination of the temperature of a given material or object, as well as of its microwave frequency reflection coefficient.
To determine the temperature of an object, it is known in the art to use measuring processes whereby the thermal noise signals emitted by this object in the microwave frequency range are picked up and a correspondence is established between strength of the signals picked up and the temperature of the object.
In this connection, the term "object" is to be taken in a very broad sense as it can refer equally well to a material object or to a material, or even to living tissues. Any absorbent body in fact emits thermal noise signals directly related to its temperature. Such thermal noise signals are emitted over a very wide frequency range.
2. Description of Background Information
To carry out temperature measurements, there are also known other processes using signals emitted in the infrared range. However, the drawback is that the signal picked up are emitted primarily by the surface of the body to be measured and the surface temperature cannot then be measured.
Another known measuring method consists in using a thermocouple, which is necessarily introduced inside the body the temperature of which one wishes to measure. However, in very many cases, the penetration of the body by the thermocouple represents a major drawback.
To avoid such drawbacks, it is preferred to make use of the thermal noise signals emitted in the microwave frequency range, that is to say frequencies ranging approximately between 0.5 and 20 GHz.
In this connection, there are known microwave radiometry devices in which the microwave radiation emitted via an antenna is picked up and the signals received are routed to signal processing means which enable the temperature of the object in question to be determined.
However, on of the main problems encountered in microwave frequency radiometry resides in the matching of the antenna in respect of the material the temperature of which one wishes to know. Indeed, the antenna used has a reflection coefficient R.sub.o and, as a result, the antenna is never perfectly matched, given that the objects to be measured generally have different configurations, sizes and properties.
Under these circumstances, the error made in measuring the temperature of the object, due to the fact that the coefficient .vertline.R.sub.o .vertline..sup.2 of the antenna is different from zero, has two implications, namely: on one hand, the emissivity of the object: =1-.vertline.R.sub.o .vertline..sup.2 is different from unity and, on the other hand, given the reflection coefficient of the antenna, a part of the noise emitted at the input of the signal processing means is reflected by the antenna, and then amplified by the said means, and thus unduly contributes to the signal measured at the output of the said means.
To remedy these different drawbacks, different processes have been devised to enable the internal temperature of a body to be measured without thereby necessitating the introduction into this body of means to detect this temperature.
Document FR-2,497,947, in fact, discloses a microwave thermography device and process based on the principle of the Dicke radiometer, using an antenna, a circulator, an auxiliary source of noise with known characteristics, an amplifier-receiver and a detector. In addition, according to this document, the use of a circulator is associated with a two-channel microwave frequency switch cyclically connecting the measuring line to the antenna or short circuiting the measuring line.
Thus, this circulator-switch assembly, on one hand, enables the signal emitted at the input of the amplifier to be absorbed and, on the other hand, enables the antenna to be presented with a temperature load substantially equal to that of the material to be measured. Under these conditions, when the coefficient .vertline.R.sub.o .vertline..sup.2 is different from zero, the reduction in noise emitted by the material to be measured is compensated for by the noise emitted by the said load and reflected by the antenna.
However, the process and device according to document FR-2,497,947 necessitate the use of a circulator, which can be disadvantageous in certain cases. The circulator is, in fact, generally formed by a ferrite element determined in accordance with the frequency range and over the size and price of which one has no control. This naturally affects, therefore, the cost of the device and on its size, precluding any possibility of monolithically integrating the device.
This being the case, document FR-2,561,769 discloses a process for controlling impedance matching in low noise reception chains and a miniature microwave thermometer for implementing this process.
Such a device comprises an antenna, a temperature and impedance adjustable standard noise source, switching means connected to the antenna and to the standard noise source, an amplifier disposed downstream of the switch, supplying a signal the amplitude of which corresponds to the difference in level between the signals from the antenna and the standard noise source, a controlled additional impedance to be placed periodically at the amplifier input, and means for analyzing the divergence between the impedances presented by the antenna and the standard noise source, to adjust, by matching their impedances, either the antenna or the noise source, to equalize the influences exerted by the additional impedance on the antenna and on the noise source, and, in consequence, to equalize the impedances presented by the antenna and the standard noise source.
The process according to FR-2,561,769 thus consists in attempting to use a reference noise source, the electronically adjustable reflection coefficient of which is made equal in modulus to that of the antenna placed in the presence of the object to be measured. For this purpose, the noise emitted by the amplifier input can be used to check the equality of the two reflection coefficients, and use is made, for this purpose, of a variable additional impedance the value of which can be electronically controlled, and which is placed at the input of the amplifier.
Such a technique makes it possible to dispense with the use of a circulator; unfortunately, however, it can only be used when the load presented by the antenna is resistive and if the length of line placed between the antenna and the amplifier is negligible.
To the disadvantages of the known devices should be added other limitations for which it is not possible to find solutions. In particular, when a circulator is used, the size of the ferrite utilized is all the greater the lower the frequency at which one operates. For example, at 1 GHz, the size soon becomes prohibitive. Furthermore, when operating to determine the temperature with a zero method, one generally attempts to make nil a factor constituted by the difference between the temperature of the auxiliary reference source and the temperature of the body to be measured. It thus becomes a delicate matter to measure the temperature of a body with a temperature of less than 273.degree. K. (.degree.C.), which is disadvantageous in certain cases.
Furthermore, in a measuring chain, the removal of the circulator element is not problem-free. Indeed, for an amplifier having a given gain g, the latter inevitably presents an input noise T.sub.e, as well as an amplifier noise T.sub.a, charactering its noise factor. Thus, in direct amplification circuits, allowances have to be made for a possible correlation between input noise T.sub.e and the output noise of the amplifier, T.sub.a when the load at the circuit input is not matched.
The measurement errors due to the mismatch depend on the refection coefficient R.sub.o , and on the said noises T.sub.e and T.sub.a.
To remedy this correlation phenomenon, it is known in the art to use in parallel on the amplifier input line aperiodic phase shifters, which make it possible to introduce randomly phases -.pi./2, +.pi./2, for example, the purpose of which is to preclude any possibility of coherent construction of the noises, and thus to cancel out any correlation between Te and T.sub.a.
However, such phase shifters reduce the amplitude of the noise signals picked up and are, furthermore, of large dimensions, which render any monolithic integration of the radiometer impossible.